1. Field of the Invention
The present invention generally relates to laser diode driving circuits, and particularly relates to a laser diode driving circuit which can precisely control a driving current for a laser diode used in printers, optical disk apparatuses, optical communication systems, etc.
2. Description of the Related Art
Semiconductor diodes are small sized and less expensive devices which can easily emit laser light by only receiving electric currents, and therefore are widely utilized in the fields of printers, optical disk apparatuses, optical communication systems, etc.
FIG. 12 shows a circuit diagram of a prior art laser diode driving circuit.
In FIG. 12, a laser diode driving circuit 100 includes a current source 101 and a current mirror circuit 104. The current source 101 changes its output current according to an input control signal SCa. The current mirror circuit 104 is formed by N-channel type MOSFETs (referred to as “NMOS transistor” hereinafter) 102 and 103 which receive a current ia from the current source 101. The laser diode driving circuit 100 further includes a current mirror circuit 107, which is formed by P-channel type MOSFETs (referred to as “PMOS transistor” hereinafter) 105 and 106 receiving an output current ib from the current mirror circuit 104. A laser diode LD is connected between a drain of the PMOS transistor 106 in the current mirror circuit 107 and a negative power supply Vss. A switch 108 is provided between a drain of the NMOS transistor 103 in the current mirror circuit 104 and a drain of the PMOS transistor 105 in the current mirror circuit 107.
In response to the control signal Sca, the current ia output from the current source 101 is mirrored by the current mirror circuit 104 and input to the current mirror circuit 107, and finally becomes a drain current for the PMOS transistor 106 in the current mirror circuit 107. The drain current from the PMOS transistor 106 is supplied as a driving current iLD to the laser diode LD, which responds to the current and emits light. The magnitude of the optical output from the laser diode LD is determined by the output current ia output from the current source 101 in response to the control signal SCa. The switch 108 is controlled by a control signal output from a controlling circuit (not shown) and controls the on and off of the laser diode LD.
There exists another prior art laser diode driving circuit which is inserted serially to a transistor controlling a current of the LD driving current, and stabilizes an optical output and lowers a power supply voltage by omitting a resistor detecting the LD driving current variation, as shown in Japanese Patent Laid-open Publication 10-270784.
However, in order to equalize an input current and output current of a current mirror circuit, a source-gate voltage Vgs of MOS transistors forming the current mirror must be the same as their source-drain voltage Vds.
FIG. 13 is a graph of an NMOS transistor, in which characteristics between its drain current id and drain voltage Vds are shown with its gate voltage Vgs being a parameter.
As shown in FIG. 13, in the saturated region, the drain current id increases as the drain voltage Vds increases, even if the gate voltage Vgs is constant. This phenomenon is known as a channel length modulation effect.
With reference to the current mirror circuit 107 shown in FIG. 12, the PMOS transistors 105 and 106 forming the current mirror circuit 107 have sources that are both connected to a positive power supply Vdd. Their gates are connected to each other, and therefore their source-gate voltages are the same. However, a drain voltage Vb varies according to the control signal SCa, that is, the current ia from the current source 101. This is because the drain voltage Vb of the PMOS transistor 105 decreases, and the anode voltage VLD of the laser diode LD increases as the driving current iLD of the laser diode LD increases. In a MOS transistor, generally, its source-gate voltage increases as its drain current increases.
While the current ia from the current source 101 is small, that is, in a B zone shown in FIG. 14A, the drain voltage Vb of the PMOS transistor 105 is larger than the drain voltage VLD of the PMOS transistor 106. As the current ia increases, the difference between the drain voltage Vb of the PMOS transistor 105 and the drain voltage VLD of the PMOS transistor 106 decreases. At a point A, the drain voltage VLD starts to exceed the drain voltage Vb.
As shown by a prior art characteristics curve in FIG. 14B, due to the channel length modulation effect, in the B zone where the current ia from the current source 101 is smaller than the current value at the point A, the current iLD supplied to the laser diode LD is larger than the target value. After the current ia goes over the point A and into a C zone, the current iLD becomes smaller than the target value, which is a problem. And also due to power supply variation, the current iLD supplied to the laser diode LD varies. The phenomenon due to the channel length modulation effect or the power supply variation similarly occurs at the current mirror circuit 104.
With reference to FIGS. 15 and 16, it is explained how to determine the currents ia, ib and iLD shown in FIG. 12.
FIG. 15 represents the B zone in FIG. 14 where the output current ia from the current source 101 is small. The current ia from the current source 101 determines an operating point N1 of the NMOS transistor 102. The gate voltage of the NMOS transistor 103 is the same as that of the NMOS transistor 102, but the drain voltage Vb of the NMOS transistor 103 is larger than the gate voltage Va. Therefore, the operation point of the NMOS transistor 103 lies at a point N2 which is shifted to the right from the gate Va. The current at the operating point N2 becomes the drain current ib of the NMOS transistor 103.
The operation point N2 is also an operation point of the PMOS transistor 105. The gate voltage Vb of the PMOS transistor 106 is the same as that of the PMOS transistor 105, but the drain voltage VLD of the PMOS transistor 106 is smaller than the gate voltage vb. Therefore, a point P2 shifted to the left becomes an operating point. The current at the operation point P2 becomes the driving current iLD for the laser diode LD. It is understood from FIG. 15 that the driving current iLD increases compared to the current ia from the current source 101.
Next, FIG. 16 represents the C zone in FIG. 14 where the output current ia from the current source 101 is large. In FIG. 16, the current ia from the current source 101 determines an operating point N1 of the NMOS transistor 102. The gate voltage Va of the NMOS transistor 103 is the same as that of the NMOS transistor 102, but the drain voltage Vb of the NMOS transistor 103 is smaller than the gate voltage Va. Therefore, the operation point of the NMOS transistor 103 lies at a point N2 which is shifted to the left from the operation point N1. The current at the operating point N2 becomes the drain current ib of the NMOS transistor 103. The operation point N2 is also an operating point of the PMOS transistor 105.
The gate voltage Vb of the PMOS transistor 106 is the same as that of the PMOS transistor 105, but the drain voltage VLD of the PMOS transistor 106 is larger than the gate voltage Vb. Therefore a point P2 shifted to the right becomes an operating point. The current at the operating point P2 becomes the driving current iLD of the laser diode LD. It is understood from FIG. 16 that the driving current iLD decreases comparing to the current ia from the current source 101. Thus, due to the channel length modulation effect, the driving current iLD for the diode LD is shifted from the target current value obtained by the control signal SCa, which is a problem.